in_array algorithm

Does anyone know which algorithm in_array() uses? For example, if I say in_array("foo", $arr) Does in_array() do an unordered sequential serach of $arr for "foo" which takes up to n comparisons [where n = count($arr)], or does it do a binary search which takes about lg(n) comparisons? Is it up to me to maintain a sorted array in the later case? Thanks, John